Abstract: The microstructure and mechanical properties of B1500HS laser tailor welded blanks before and after hardening heat treatment were studied by optical microscopy, scanning electron microscopy, energy dispersive spectrometry, uniaxial tensile testing, and Vickers hardness testing. Before the heat treatment, the strength of the laser tailor welded blanks is much higher than that of the base metal, and the hardness is very unevenly distributed. After the heat treatment, the element distribution of the laser tailor welded blanks has almost no change, the overall strength is greatly improved, the plasticity greatly declines, the lateral plastic is the worst, and the hardness from the weld to base metal smoothly changes. This smooth transition of hardness makes the stress and strain distribution more uniform, ensures the good continuity of mechanical properties between the base material and the weld, and can significantly improve the formability of the laser tailor welded blanks.